A) Field of the Invention
The present invention relates to a method of making superabsorbent polymer foams and, more particularly, to methods of making an improved superabsorbent polymer foam having a morphology which provides improved absorptive properties, e.g., substantially continuous intercommunicating channels substantially throughout the foam and a relatively high surface area to mass ratio. The superabsorbent polymer foams of the present invention are especially useful in absorbent members which can be incorporated into absorbent articles such as diapers, adult incontinence pads, sanitary napkins, and the like.
B) Background Information
"Superabsorbent" polymer materials (also known as hydrogels, hydrocolloids, osmotics and absorbent gelling materials) are generally capable of absorbing large quantities of fluids such as water and body exudates and are further capable of retaining such absorbed fluids under moderate pressures. These absorptive characteristics make such materials especially useful for incorporation into absorbent articles such as diapers, sanitary napkins, and the like.
The art teaches superabsorbent foams formed of superabsorbent polymer material. For example, U.S. Pat. No. 4,529,739 issued to Scott et al. on Jul. 16, 1985 and U.S. Pat. No. 4,649,164 issued to Scott et al. on Mar. 10, 1987, teach a foamed, water-swellable, polymeric water absorbent material prepared by contacting a polymer capable of having a water-swellable character and containing acid moieties in acid form with a blowing agent capable of neutralizing the acid moieties. U.S. Pat. No. 4,808,637 issued to Boardman et al. on Feb. 28, 1989 and European Patent Application No. 0295438 published by Boardman on Dec. 21, 1988, teach an acrylate superabsorbent composition having an improved rate of absorbency, low residual acid content and a low acrylate monomer content, formed by uniformly reacting a mixture of acrylic acid, an alkali metal salt of carbonic acid, aluminum acetate, sodium persulfate and water using a microwave heat source. U.S. Pat. No. 5,154,713 issued to Lind on Oct. 13, 1992, teaches a superabsorbent polymer having an increased rate of water absorption obtained by the addition, preferably prior to polymerization, of a carbonate blowing agent to a monomer solution of the monomers used to form the superabsorbent polymer.
The art also teaches foams containing particulate, non-foamed superabsorbent materials. See, for example, U.S. Pat. No. 4,394,930 issued to Korpman on Jul. 26, 1983; U.S. Pat. No. 4,415,388 issued to Korpman on Nov. 15, 1983; and Great Britain Patent Application 2136813A published by Korpman on Sep. 26, 1984. These references teach foam products prepared from solid, particulate superabsorbent polymer, a blowing agent, and a liquid polyhydroxy compound. U.S. Pat. No. 4,725,629 issued to Garvey et al. on Feb. 16, 1988, teaches a superabsorbent polyurethane foam based on an interpenetrating polymer network of a crosslinked polyurethane and crosslinked addition polymer, prepared by forming the polyurethane foam in the presence of addition polymerizable monomers, a crosslinking agent and a free radical initiator.
While the above foams may be useful absorbent materials, they have not shown to be optimal for use in disposable products because their absorptive properties tend to be limited. The superabsorbent foams formed of superabsorbent material tend to be characterized by discontinuous channels. Such foams tend to possess a relatively large average cell size and wide cell size distribution, i.e., the foams have a relatively large diameter capillary structure in which the capillary diameter varies widely and randomly. It is believed that the large average cell size and discontinuous channels result in a foam product having a relatively low surface area to mass ratio such that the osmotic absorptive rate is not optimal. In addition, it is believed that the discontinuity of foam channels and a wide cell size distribution limit the ability of fluids to flow by capillary transport through the foam structure. This limitation of the capillary transport properties is believed to limit both the osmotic and capillary absorptive properties of such foams, more particularly the capillary absorptive capacity and rate and the osmotic absorptive rate. Thus, the absorptive rates and capacities of such foam products tend to be limited.
In a composition in which particulate superabsorbent polymer materials are present in a foam made of non-superabsorbent material such as polyurethane, the foamed portion of the composition is not superabsorbent as defined herein since it does not have sufficient ability to retain absorbed fluids. Therefore, although the superabsorbent particulate portion of the structure may be able to retain fluids, the overall capacity of the foam to absorb and retain fluids is limited. Further, the overall absorptive properties of the foam tend to be limited due to the relatively low surface area to mass ratio of the particulate portion relative to the foam portion. This is believed to be particularly important for absorption of body fluids containing high molecular weight components, e.g., blood and menses. Such components are believed to deactivate particulate superabsorbent materials due to their large molecular size relative to the particulate material. The high molecular weight components may also deactivate regions of a foam characterized by a discontinuous channel. In addition, the use of polyurethane materials in applications intended for human contact such as diapers presents concerns over the potential toxicity of materials that may be used to prepare the polyurethane.
Thus, there is a continuing need in the field of superabsorbent materials to provide materials having faster absorptive rates and greater absorptive capacities. More particularly, there is a need to provide absorbent articles having such improved absorptive properties while reducing the overall thickness of the absorbent article. For absorbent articles, it is desirable to use materials which are non-toxic to humans. Additionally, it is desirable to provide such materials and absorbent articles in a cost-effective manner, e.g., by rapid, simple and safe manufacturing techniques.
It is therefore an object of the present invention to provide a superabsorbent polymer foam having an improved absorptive rate and improved fluid distribution properties. An additional object of the present invention is to provide a superabsorbent polymer foam having a morphology which provides an increased absorptive rate, particularly characterized by a high surface area to mass ratio. Another object of the present invention is to provide such a superabsorbent polymer foam which does not present undesirable risks to human health. In addition, it is an object of the present invention to provide a cost-effective method of making such foams.
An additional object of the present invention is to provide absorbent products, members and articles having improved absorptive rates. Another object of the present invention is to provide absorbent members whose absorptive properties are tailored to the requirements of their intended application. An additional object of the present invention is to minimize the thickness of absorbent articles while providing improved absorptive properties.